During the past year we have collaborated with Katy Rezvani, Stephan Rezvani and John Barrett in studies of suppressor T cell engraftment after allotransplantation. These studies exploit the recent development of reliable flow cytometric techniques for distinguishing between activated human CD4 effector (CD4+,CD25+, FoxP3-) T cells and CD4 regulatory (CD4+, FoxP3+) T cells to gain additional insights into the regulation of alloreactivity after allogeneic stem cell transplantation. In comparing cell subtypes in leukocyte samples stored before and after transplant, it was noted that stem cell transplant recipients who will develop acute graft versus host disease (GVHD) have increased numbers of circulating effector CD4 cells and decreased numbers of regulatory CD4 T cells in their blood by day 30 post transplant compared to patients without GVHD. Based on chimerism engraftment studies, we could be sure almost all of these cells were donor in origin. The studies support the emerging concept that both effector and regulatory T cells can expand extensively during engraftment after stem cell transplantation. In this model, the balance between these two populations is critical in determining the extent of post-transplant GVHD. Of practical importance, the regulatory T cell concentration in the donor graft appeared to be a statistically significant predictor of clinical outcome i.e. a high concentration of regulatory cells in the graft was associated with a lower incidence of GVHD. These findings suggest new approaches for manipulating donor graft content to prevent GVHD.